1. Field of the Invention
The present invention is directed to a high voltage, highly conductive electrolyte for use in electrolytic capacitors and to an electrolytic capacitor impregnated with the electrolyte of the present invention for use in an implantable cardioverter defibrillator (ICD).
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators, such as those disclosed in U.S. Pat. No. 5,131,388, incorporated herein by reference, typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size, reliability and ability to withstand relatively high voltage. Conventionally, such electrolytic capacitors include an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. While aluminum is the preferred metal for the anode plates, other metals such as tantalum, magnesium, titanium, niobium, zirconium and zinc may be used. A typical solvent-based liquid electrolyte may be a mixture of a weak acid and a salt of a weak acid, preferably a salt of the weak acid employed, in a polyhydroxy alcohol solvent. The electrolytic or ion-producing component of the electrolyte is the salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, comprising a planar, layered, stack structure of electrode materials with separators interposed therebetween, such as those disclosed in the above-mentioned U.S. Pat. No. 5,131,388.
The capacitance of an electrolytic capacitor is provided by the anodes. The paper separator and the cathode serve important roles in realizing the full capacitance of the anodes. A clear strategy for increasing energy density in the capacitor is to minimize the volume taken up by the paper and cathode foil and maximize the number of anodes, thus reducing the size of the device. This may be achieved by using a multi-anode stack configuration. For example, a multi-anode stack consists of a number of units of: a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode; with neighboring units sharing the cathode between them. However, to charge and discharge the inner anodes (furthest from the cathode) charge must flow through the outer anodes. With typical anode foil, the path through an anode is quite tortuous and results in a high equivalent series resistance (ESR) for a multi-anode configuration. Thus, ESR increases as more anodes are placed together in the stack. To combat this problem, it has been suggested to provide very low resistivity electrolytes which may be used in a multi-anode configuration without an excessive ESR increase.
U.S. Pat. No. 5,111,365 to Dapo discloses an electrolytic capacitor provided with a low resistivity electrolyte. The disclosed electrolytic capacitor consists of aluminum anode and cathode members separated by an insulating spacer impregnated with an electrolyte consisting of a solution containing, (1) 50%-70% by weight of N-methylformamide; (2) up to 30% by weight of 2-methoxyethanol, 2-ethoxyethanol, ethylene glycol or 1,2-propylene glycol; (3) 12-20% by weight of an aromatic dicarboxylic acid selected from the group consisting of isophthalic acid and terephthalic acid; (4) from 4%-10% by weight of dimethylamine or monomethylamine, the ratio of the amine to the dicarboxylic acid being less than 2.00:1 and greater than 1.67:1; (5) up to 0.5% by weight of pelargonic acid; (6) up to 0.1% by weight of phosphoric acid; and (7) up to 8% by weight of water. These capacitors have been found to be useful at relatively low voltage applications, for example about 55 VDC.
U.S. Pat. No. 5,519,567 to Dapo discloses an electrolytic capacitor having aluminum anode and cathode members separated by a paper insulating spacer impregnated with an electrolyte solution containing (1) 1.50-4.00 wt. % of pelargonic acid; (2) 0.00-80.00 wt. % of N-methylformamide; (3) up to 0.05 wt. % of phosphoric acid; (4) 7.00-25.00 wt. % of isophthalic acid or an equivalent amount of terephthalic acid; (5) 1.50-15.00 wt. % of water; (6) an aliphatic amine sufficient to provide a pH of 7.2-8.5 and (7) ethylene glycol in an amount of about 0.00-70.00 wt. % of the solvent present, the mole % of the pelargonic acid being not greater than 5.5 mole % of all the acids present. Such a capacitor has been found particularly useful for the low-volt range of 0-100 VDC.
While these references disclose low resistivity electrolytes which may be used for low voltage capacitors, what is needed in the art is an electrolyte that provides low equivalent series resistance in an electrolytic capacitor operating at 400 volts, the useful energy for capacitors in implantable cardioverter defibrillators.
The present invention is directed to a high voltage, highly conductive electrolyte for use in electrolytic capacitors and to an electrolytic capacitor impregnated with the electrolyte of the present invention for use in an implantable cardioverter defibrillator (ICD). The electrolyte according to the present invention is composed of a two solvent mixture of ethylene glycol and N-methylformamide; a combination of hypophosphorous acid, boric acid and an aliphatic dicarboxylic acid of carbon chain length from eight to twelve, such as azelaic, sebacic, or brassylic acid; an amine including ammonia, ammonium hydroxide, diethylamine, dimethylamine, triethylamine, or triethanolamine; and a nitro-substituted aromatic compound as a degassing agent, such as 3xe2x80x2-nitroacetophenone. Anhydrous ammonia may also be added to neutralize the solution. A representative composition according to the present invention that displays the desired properties is: 82.1% by weight ethylene glycol, 4.1% by weight N-methylformamide, 0.2% by weight hypophosphorous acid, 5.7% by weight azelaic acid, 1.0% by weight boric acid, 1.0% by weight 3xe2x80x2-Nitroacetophenone, 1.5% by weight ammonium hydroxide (28-30% w/w), 0.8% by weight anhydrous ammonia and 3.6% by weight water.
In an alternative embodiment of the electrolyte of the present invention, the ethylene glycol/NMF two solvent mixture can be substituted with 1,2-propanediol, using xcex3-butyrolactone as a cosolvent with or without NMF. A representative composition according to this embodiment of the present invention that displays the desired properties is: 55.5% by weight 1,2-propanediol, 21.1% by weight xcex3-butyrolactone and 12.0% by weight N-methylformamide (NMF), 5.0% azelaic acid, 0.8% boric acid, 0.8% 3xe2x80x2-Nitroacetophenone, 2.3% by weight triethylamine and 2.5% water.
The electrolyte according to the present invention produces a low ESR multi-anode capacitor when combined with appropriate cathodes, spacers and etched and formed anodes at 400 volts, the useful energy for capacitors in implantable cardioverter defibrillators This electrolyte will allow the construction of multi-anode capacitors with low equivalent series resistance (ESR), which will have superior energy density and delivery, while being thinner due to fewer spacer and cathode layers with the same number of anodes. This resultant capacitor, when incorporated into an ICD, will reduce the size and thickness of the overall device, and allow for greater patient comfort, in addition to superior therapy.